The invention relates to a pressure sensor and a method of monitoring its operation.
To be more precise, the invention relates to a pressure sensor with foresighted maintenance control and monitoring for diaphragm fracture, and to a method for monitoring for diaphragm fracture.
During pressure measurement, a force generally acts on the surface of an elastic deformation body, a signal being produced as a function of its deformation, this signal being a measure of the applied pressure.
In capacitive pressure sensors, a movable electrode of a capacitor, for example, is displaced relative to a fixed counter-electrode by the deflection of the deformation body, and this produces a measurable change in capacitance. Planar deformation bodies such as diaphragms, plates or circular disks are especially suitable for capacitive pressure sensors, the movable electrode being applied as a coating to the planar deformation body and being arranged at a distance d parallel to the counter-electrode. In this case, pressure sensors with disks made of brittle materials, preferably ceramic disks, have proved especially successful as deformation bodies.
For certain applications, it is advantageous if the process medium does not act directly on the actual pressure sensor for producing the measuring signal but rather acts on an upstream deformation body, the deformation of which is then transmitted by means of a transmission liquid to the actual pressure sensor via a closed line system. In this case, the upstream deformation bodies are in particular metallic diaphragms.
However, these diaphragms and their joints (welds) may become porous and finally fracture on account of aging processes or corrosion, a factor which leads to contamination of the process medium with the transmission liquid. The defect described would not be noticed by the pressure sensor, since the process pressure continues to act on the actual sensor. Furthermore, there is the risk of a a bacterial center forming at the site of fracture, which is unacceptable in particular for the foodstuffs industry.
In order to avoid the consequences described, the diaphragms must be monitored in a preventative manner at regular intervals, regardless of their actual state, and if need be exchanged, This leads to an increased maintenance outlay with corresponding downtimes and costs associated therewith.
The object of the invention is to improve the conventional pressure sensors with upstream diaphragms or deformation bodies to the effect that actual aging or a fracture of the diaphragms or deformation bodies can be detected in good time.
This object achieved by the pressure-measuring device including a housing which ha a passage having a first opening and a first deformation body, which is exposed to the measuring medium and closes off the first opening and a second opening and a second deformation body, which closes off the second opening, and a transmission liquid for introducing pressure from the first deformation body to the second deformation body, a pressure sensor which based on the deflection body, produces a measuring signal, and means for monitoring a material property of the transmission liquid, by the method of recording at least one measured variable which gives an indication of the current value of a material property of the transmission liquid, of comparing the at least one recorded measured variable or a variable derived therefrom with at least one reference value, and of producing an alarm signal if the comparison results is a deviation or deviations from a reference value(s), by the program for operating on an electronic data processing system for carrying out the noted method.
The invention is based on the consideration that, in the event of a diaphragm fracture, not only does the transmission liquid escape from the pressure-measuring device, but the process medium in like manner penetrates into the pressure-measuring device and contaminates the transmission liquid. This leads to a measurable change in the characteristic properties of the liquid in the pressure-measuring device. A diaphragm fracture or a porous diaphragm can thus be verified by monitoring a suitable material property of the transmission liquid.
The relative dielectric constant xcex5r is especially suitable for monitoring capacitive sensors. Other suitable material properties are the electrical conductivity, the thermal conductivity, the viscosity, or the absorptive capacity for electromagnetic radiation, in particular light of a certain wavelength, or the refractive index at a certain wavelength, the selection of a material property depending on the measuring principle of the respective pressure sensor.
The invention is suitable both for devices for measuring absolute pressure and relative pressure and for devices for differential-pressure measurement.
The pressure sensor is preferably a capacitive pressure sensor, in particular having a ceramic deformation body, or a polysilicon cell.
The sensor elements for the pressure measurement may be used for measuring the characteristic properties, or additional sensor elements may be provided.
In a capacitive sensor, the dielectric constant xcex5r, for example, between the electrodes for the pressure measurement may be determined, or additional electrodes may be provided.
The invention also relates to a method of monitoring for diaphragm fracture for a pressure-measuring device in which the process pressure is transmitted by means of a transmission liquid from a first deformation body designed as a diaphragm to a second deformation body.
In an especially preferred manner, the material property taken into account is the relative dielectric constant of the transmission liquid.
The invention also comprises a program for operating on an electronic data processing system for carrying out the method according to the invention.